What year did they stop making half dollars out of pure silver?

US halves were never made of pure silver, all pre-1965 (1964 was the last) coins are 90% silver and 10% copper. From 1965 to 1970 they were made of 40% silver and 60% copper. From 1971 to the present they are 75% copper and 25% nickel bonded to an inner core of pure copper.